Ur-Quan Masters (aka. Star Control 2)
But, it's not really abandoned anymore. The developers are FINALLY making an official sequel!
The sequel is not open source, but UQM/SC2 is.
This happened in the early 2000s, but I think they found the source code to a port of the game and said "We haven't earned any money from sales of this game in a decade [and buying digital games wasn't really a thing yet, as people generally believed that anything digital shouldn't have a price], so let's release this to the community to open source as long as they do all the reverse porting and support!"